The US Drug Enforcement Administration (DEA) has announced it will probe the doctors who gave the Australian actor the drugs which, when taken together, shut down his body until he died.
Source : Herald Sun

Earlier media reports indicated that some of the prescriptions were from doctors in Europe or the UK where he was doing a film.

That’s a danger of a mobile lifestyle and seeing different doctors all the time. They don’t know what the last doctor prescribed.

And patients don’t always tell doctors that they’re already on (or have) several medications. Either because they really don’t recall the last set of pills they got, or because they lie and simply want more for existing addictions. My wife (a nurse at a large diagnostic clinic) sees that a lot…people trying to get narcotics.
I’m not saying that’s what happened with Ledger, but it really does happen.

At the end of the day though..if Ledger had all these legal pills in his possession,
you would have thought he’d be smart enough not to mix them or take too many at one time.
It’s not like this has never happened before.

Sad case. RIP.